Sigurjón Jónsson

Fault Slip Distribution of the 1999 Mw 7.1 Hector Mine, California, Earthquake, Estimated from Satellite Radar and GPS Measurements

We use a combination of satellite radar and GPS data to estimate the slip distribution of the 1999 M w 7.1 Hector Mine Earthquake, a right-lateral strikeslip earthquake that occurred on a northwest–southeast striking fault in the southern California Mojave Desert. The data include synthetic aperture radar interferograms (InSAR) from both ascending and descending orbits, radar amplitude image offset fields (SARIO) for both ascending and descending azimuth directions, and campaign GPS observations from 55 stations provided by Agnew et al. (2002). We model the fault with nine segments derived from the field-mapped fault rupture, the SARIO data, and aftershock locations. We first estimate the dip of each fault segment, as well as a single constant strike-slip component across each segment, resulting in an average dip of 83° to the northeast and slip of up to 5.6 m. Then, we fix the optimal fault segment dip, discretize the fault segments into 1.5 km × 1.5 km patches, and solve for the variable slip distribution using a nonnegative least-squares method that includes an appropriate degree of smoothing. Our preferred solution has both right-lateral strike-slip and reverse faulting. The estimated geodetic moment is 5.93 × 10 19 N m ( M w 7.1), similar to seismological estimates, indicating that there are insignificant interseismic and postseismic deformation signals in the data. We find strike-slip displacements of up to 6.0 m and reverse faulting of up to 1.6 m, with the maximum slip located just northwest of the epicenter. Most of the slip is concentrated northwest and south of the epicenter; little slip is found on the northeastern branch of the fault. The SARIO data and our modeling indicate that the amount and extent of surface fault rupture were underestimated in the field.

Fault slip distribution of two June 2000 MW6.5 earthquakes in South Iceland estimated from joint inversion of InSAR and GPS measurements

Abstract We present the first detailed estimates of co-seismic slip distribution on faults in the South Iceland Seismic Zone (SISZ), an area of bookshelf tectonics. We have estimated source parameters for two M W 6.5 earthquakes in the SISZ on June 17 and 21, 2000 through a joint inversion of InSAR and GPS measurements. Our preferred model indicates two simple 15 km long, near vertical faults extending from the surface to approximately 10 km depth. The geometry is in good agreement with the aftershock distribution. The dislocations experienced pure right-lateral strike-slip, reaching maxima of 2.6 m and 2.9 m for the June 17 and 21 events, respectively. We find that the distribution of slip with depth may be correlated to crustal layering, with more than 80% of the total geometric moment release occurring in the uppermost 6 km. According to the distributed slip model the middle and upper crust appears to be more apt to generate large displacements than the lower crust. The geodetic estimates of seismic moments are 4.4×10 18 Nm ( M W 6.4) and 5.0×10 18 Nm ( M W 6.5). The total moment released by the two events equals that generated by several decades of plate motion in the area, but is only a fraction of the moment accumulated in the area since the last major earthquake in 1912.

Effects of subglacial geothermal activity observed by satellite radar interferometry

We use one day Synthetic Aperture Radar (SAR) interferograms from data of the Earth Remote Sensing Satellites ERS-1 and ERS-2 to study ice flow and uplift of two surface depressions within the Vatnajokull ice cap, Iceland. The ice cauldrons are created by melting at sub-glacial geothermal areas. Meltwater accumulates in a reservoir under the cauldrons over 2 to 3 years until it drains in a jokulhlaup under the ice dam surrounding the reservoir. The ice surface in the depressions drops down by several tens of meters during these draining events but rises again, as ice flows into the depressions, until a jokulhlaup occurs again. Using SAR interferograms we quantify an uplift rate of about 2 to 18 cm/day within the jokulhlaup cycle varying with the surface slope of the depressions. The uplift rate is high during the first months after a jokulhlaup when the cauldron is relatively deep with steep slopes, but the uplift rate decreases as the cauldron is gradually filled. A simple axisymmetric model simulating the ice-flow into one of the depressions describes quantitatively the filling rate of the cauldron and qualitatively the shape of the ice flow field. The best-fit model has an ice flow law parameter A0 that is about one order of magnitude lower than typically estimated for temperate glaciers.

Low rates of deformation of the Furnas and Fogo Volcanoes, São Miguel, Azores, observed with the Global Positioning System, 1993-1997

Abstract We report results of Global Positioning System (GPS) measurements at the Furnas and Fogo (Agua de Pau) Volcanoes, Sao Miguel island, Azores. GPS surveys were carried out in 1993, 1994, and 1997. The GPS network consists of 16 stations that are mainly distributed in the vicinity of the Furnas Volcano. The measurements indicate slow-progressing ground deformation of the Furnas Volcano in 1993–1997. We propose two alternative hypotheses to explain the observations. The first one assumes a source of inflation located northwest of the Furnas caldera, since there part of the GPS network seems to expand. The second hypothesis invokes a combination of two processes as being responsible for the observed deformation pattern; plate divergence between the Eurasian and African plates together with a deflation of the Furnas caldera. The inferred plate boundary deformation zone is narrow with 75% of the 4 mm/yr full plate spreading probably being accommodated over the 10-km width of Sao Miguel island. GPS stations at Fogo show displacements towards its caldera, indicating slight deflation of the volcano, possibly due to a pressure decrease in a shallow magma chamber or to extraction of water and steam from the ground by a nearby geothermal power plant. Further geodetic observations are needed, spanning a longer time interval, in order to distinguish between different hypotheses.